In general, webs such as thin polyester foils or other sheet materials are manufactured in a continuous process and the final products are wound up on rolls for storage and transportation.
During the operation of winding the web on a roll, it is wished to ensure an homogeneous winding on the roll (i.e. without wrinkles or puckers) and to trap as less as possible air between each web layer on the roll.
The problem is particularly acute for (ultra) thin films with thickness as low as the micron size and speeds up to 1000 m/min.
In the prior art, webs, especially in case of thin ones, are usually wound at high velocities (i.e. more than a few hundred meters per minute) with the help of a nip roller (also called packroll) to prevent excessive air entrainment.
In p. 33 to 35 of Air Entrainment with A Forced-Loaded Nip Roller, Y. Bae Chang, F. W. Chambers, J. J. Shelton, Web Handling Research Center, Oklahoma State University, May 1994, it is taught that:                (aa) to keep the amount of air entrainment under a certain level at high speed operation, the most effective way is to reduce the diameter of packroll;        (bb) if the packroll (or its covering) is softer than the winding roll and too much air is entrained, then the problem can be solved by using harder materials for the packroll;        (cc) the amount of entrainment air is not very effectively reduced by increasing the nip loading and if said loading is increased too much, other winding problems can occur.        
Furthermore, this document teaches that there may be practical problems or limitations in reducing the size of packrolls, for example, the packroll may become too flexible if it is too thin. However, it suggests to design slender packrolls because of its importance in air entrainment and gives two examples of possible design changes by way of schematic drawings. A first drawing shows a slender roll between a roll and a winding roll, the web passing from the roll to the slender roll and then to the winding roll. A second drawing shows a slender roll between two rolls and a winding roll, the web passing from one of those rolls to the slender roll and then to the winding roll.
However, this document does not give enough hints for to put those principles into practice, i.e. there are several practical problems that are neither solved nor mentioned. A first problem is to ensure the correct position of the slender roll between the roll(s) and the winding roll since the slender roll becomes flexible due to its low diameter. Another problem is to ensure that the tangential speed of the slender roll and of the rolls is identical at each point there between over their length in order to avoid friction on the web. Another problem is to ensure the spreading of the web before winding it on the winding roll, i.e. wrinkles may remain on the web once wound on the winding roll. A further problem is to allow an easy initiation of the winding of the web: the difficulty consists in passing the web between the roll and the slender roll and between the slender roll and the winding roll. Another further problem is to apply a pressure distribution over the width of the winding roll that results in a uniform air exclusion.
The purpose of the present invention is to provide an apparatus and a method for winding webs on winding rolls, which overcome these problems.